WASHINGTON DC (CBS 11 NEWS) – One day before Aaron Alexis walked into Washington’s naval yard where he gunned down 12 people, he legally rented a rifle and purchased a shotgun and 24 shells from a Virginia gun store, just 15 minutes away from the scene of the crime.

According to the FBI, Alexis was able to gain access to the navy yard with a valid ID Monday morning before he opened fire. Documents obtained by CBS show Alexis bought the gun on Sunday. It’s still unclear why he decided to target the navy yard.

Two months before the shootings, CBS-11 has learned Alexis reported hearing voices. A police report from Newport, Rhode Island shows the former Naval Reservist called officers claiming he kept hearing the voices of three people who he worried were out to hurt him. The Associated Press reports Alexis had recently started receiving treatment from the V-A for paranoia and a sleep disorder. But even so, Alexis, an employee working for a naval subcontractor, still retained his valid pass and security clearance, which allowed him into the high security facility.

“We continue to look into Mr. Alexis’s past, including his medical and criminal histories,” said Valerie Parlave, assistant director in charge, FBI Washington Field Office, at a Tuesday afternoon news conference.

The FBI says Alexis acted alone. He shot and killed 12 people, before he died in a gun battle with police.

“We do not have any information at this time that he had an ar-15 in his possession,” said Parlave. “We also believe that Mr. Alexis may have gained access to a handgun once inside the facility and after he began shooting.”

Police say Alexis intended to kill as many people as possible and credit quick police response for saving lives.

“Literally two minutes after the call was dispatched, we had officers at the gates arriving on the scene. Within seven minutes had officers at the building entering the building to engage an active shooter,” said Washington D.C. Police Chief Cathy Lanier.

Early Tuesday, the FBI towed Alexis’ Toyota Prius from the Washington navy yard as they continue to search for a motive behind the attack. Investigators say Alexis came to Washington on August 25 and stayed at local hotels leading up to the mass shooting. He checked into a “Residence Inn” Hotel on September 7.

Defense Secretary Chuck Hagel was among those who laid a wreath in honor of those killed in Monday’s rampage. At the Washington Nationals baseball game near the navy yard, the team and fans held a moment of silence in honor of the victims.

The following is a list of the twelve people killed in the shooting. They range in age from 46 to 73 years old, and worked an array of jobs in the naval yard. One was a handyman, another designed ships for the navy. They leave behind several spouses, children and grand children: